1. Field of the Invention
The present invention relates to a magnetic recording medium such as a magnetic disc and a magnetic drum. More particularly, it relates to a magnetic medium in which the non-magnetic layer is a nickel-copper-phosphorus layer containing 20 to 65% by weight of copper, in which the layer is formed by electroless plating.
2. Description of the Prior Art
According to the conventional technology, magnetic recording media such as magnetic discs are produced by forming a non-magnetic layer on a non-magnetic substrate such as aluminum and then forming a magnetic layer thereon. The non-magnetic layer is usually a nickel-phosphorus (Ni-P) layer of comparatively high phosphorus content formed by electroless plating.
The Ni-P layer is deposited from an electroless nickel plating solution containing a hypophosphite as a reducing agent. It remains non-magnetic as long as it is kept at room temperature or is left as is after deposition; but it becomes magnetized when it is heated to more than about 200.degree. C. This property causes a problem for the manufacture of the magnetic recording medium. Although heating of the non-magnetic Ni-P layer up to 200.degree. C., especially 100.degree. C. does not cause serious problems, the non-magnetic Ni-P layer is exposed to temperatures above 200.degree. C. when the magnetic layer is being formed thereon by sputtering during the process of manufacturing the magnetic recording medium, which results in the magnetization of the non-magnetic Ni-P layer. The magnetization of the Ni-P layer adversely affects the performance of the magnetic recording medium.
A countermeasure worked out to solve this problem is to increase the content of phosphorus in the Ni-P layer up to 10% and above, so that the magnetization is minimized. However, this does not bring about a satisfactory solution because the reduction of magnetization by the increase of phosphorus content is a matter of relativity. The Ni-P layer of high phosphorus content still becomes magnetized. Moreover, it is difficult to deposit the Ni-P layer of high phosphorus content in the stable manner.